1. Field of the Invention
The present invention relates to a method of forming a pattern having an optional angle in a charged particle exposure system.
2. Description of the Related Art
In recent years, integrated circuits become more highly more integrated with more functions and are widely used in computers, telecommunications equipment, machine controllers, etc., in all industrial fields.
The high integration and improved functions of the ICs are due to improvements in fine processing techniques, particularly an optical exposure technique which can achieve a precision of submicron order. It is, however, difficult for the optical exposure technique to open very small electrode windows and realize a positioning accuracy of less than 0.15 micrometers.
For such fine processing with a precision of less than 0.15 micrometers, a charged particle exposure technique such as an electron beam exposure technique is employed.
In the charged particle exposure technique, a conventional pattern generating method using a variable rectangular beam deals only with patterns of rectangles, isosceles right triangles, and parallelograms of 45 degrees.
To further integrate a memory such as a DRAM, the area of each memory cell must be reduced. It is, however, impossible to reduce areas with patterns of rectangles and non-rectangles (triangles, parallelograms, and trapezoids). It is, therefore, required to write patterns having optional angles.
A conventional method of generating patterns having optional angles other than 45 degrees will be explained with reference to FIG. 9 which shows a standard type electron beam exposure system.
At first, a pattern designer prepares many patterns having optional angles other than 45 degrees, using CAD, etc. The prepared patterns are stored in a memory such as a magnetic tape 53 and a magnetic disk 52. In accordance with an instruction from a computer 51, a necessary pattern is selected from the memory and stored in a data memory 55. The pattern stored in the data memory 55 is supplied to an exposure unit 58, 61 through a pattern generator 56 and a pattern correcting circuit 57. These operations take place for each pattern so that it may take a long time from the step of selecting patterns to the step of exposing. Since it is necessary to prepare and store all necessary patterns (particularly triangular patterns) having necessary angles, this method requires a very large memory capacity. Since the number of triangles having optional angles is extremely large, the exposure system must have a complicated controlling system of large circuit scale.